Method for the production of flexographic printing forms by means of electron beam cross-linking and laser engraving

ABSTRACT

A method for the production of flexographic printing forms by means of laser engraving, wherein at least one elastomer relief layer is applied to a dimensionally-stable carrier. The relief layer comprises at least one elastomer binding agent and at least one absorber for laser radiation; the relief layer is entirely cross-linked by means of electron radiation at a minimum overall dose of 40 kGy; a printed relief is engraved into the cross-linked relief layer by means of a laser. The invention also relates to flexographic printing forms which can be obtained according to said method.

[0001] The present invention relates to a process for the production offlexographic printing plates by means of laser engraving by applicationof at least one elastomeric relief layer to a dimensionally stablesubstrate, the relief layer comprising at least one elastomeric binderand at least one absorber for laser radiation, uniform crosslinking ofthe relief layer by means of electron beams in a minimum total dose of40 kGy and engraving of a printing relief into the crosslinked relieflayer by means of a laser. The present invention furthermore relates toflexographic printing plates obtainable by the process.

[0002] In the direct laser engraving technique for the production offlexographic printing plates, a relief suitable for printing is engraveddirectly into a relief layer suitable for this purpose. The engraving ofrubber impression cylinders by means of lasers has been known inprinciple since the late 60s. However, this technique did not attractwider commercial interest until recent years with the arrival ofimproved laser systems. The improvements in the laser systems includebetter focusability of the laser beam, higher power andcomputer-controlled beam guidance.

[0003] Direct laser engraving has several advantages over theconventional production of flexographic printing plates. A number oftime-consuming process steps, such as creation of a photographicnegative or development and drying of the printing plate, can bedispensed with. Furthermore, the sidewall shape of the individual reliefelements can be individually formed in the laser engraving technique.Whereas in photopolymer plates the sidewalls of a relief dot divergecontinuously from the surface to the relief base, a sidewall which isperpendicular or virtually perpendicular in the upper region andbroadens only in the lower region can also be engraved by means of laserengraving. Consequently, there is little or no increase in tonal valueeven with increasing wear of the plate during the printing process.Further details of the laser engraving technique appear, for example, inTechnik des Flexodrucks, page 173 et seq., 4th Edition, 1999, CoatingVerlag, St. Gallen, Switzerland.

[0004] In principle, commercial photopolymerizable flexographic printingelements can be used for the production of flexographic printing platesby means of laser engraving. U.S. Pat. No. 5,259,311 discloses a processin which the flexographic printing element is photochemicallycrosslinked by uniform exposure in a first step and a printing relief isengraved by means of a laser in a second step.

[0005] EP-A 640 043 and EP-A 640 044 disclose single-layer andmultilayer elastomeric laser-engravable recording elements,respectively, for the production of flexographic printing plates. Theelements consist of reinforced elastomeric layers. For the production ofthe layer, elastomeric binders, in particular thermoplastic elastomers,for example SBS, SIS or SEBS block copolymers, are used. As a result ofthe reinforcement, the mechanical strength of the layer is increased inorder to permit flexographic printing. The reinforcement is achievedeither by introduction of suitable fillers, photochemical or thermochemical crosslinking or combinations thereof.

[0006] A precondition for the production of flexographic printing platesby means of laser engraving is that the laser radiation is firstabsorbed by the relief layer. Below a specific threshold energy whichmust be introduced into the relief layer, no engraving is in generalpossible. Above the threshold energy, the speed or efficiency of theengraving depends on the energy absorbed per unit time. The absorbanceof the relief layer for the laser radiation chosen in each case shouldtherefore be as high as possible.

[0007] In the laser engraving of flexographic printing elements, largeamounts of material must be removed. Powerful lasers are thereforerequired. CO₂ lasers having a wavelength of 10 640 nm can be used forthe laser engraving of flexographic printing plates. Very powerful CO₂lasers are commercially available. The elastomeric binders which areusually used for flexographic printing plates generally absorb radiationhaving a wavelength in the region of about 10 μm. They can in principletherefore be engraved using CO₂ lasers (wavelength of 10 640 nm), asdisclosed, for example, by U.S. Pat. No. 5,259,311, even if theengraving speed is not always optimum. Furthermore, the achievableresolution and hence the quality of the printing plate on engraving withCO₂ lasers are limited. In addition to physical limits which exist inany case, the beam becomes increasingly difficult to focus withincreasing power.

[0008] Solid-state lasers having wavelengths in the region of 1 μm canalso be used for the laser engraving of flexographic printing elements.For example, powerful Nd-YAG lasers (wavelength 1 064 nm) can be used.Compared with CO₂ lasers, Nd-YAG lasers have the advantage thatconsiderably higher resolutions are possible owing to the substantiallyshorter wavelength. In general, however, elastomeric binders offlexographic printing plates do not absorb the wavelength of solid-statelasers or do so only poorly.

[0009] It has been proposed that substances absorbing IR radiation bemixed with the relief layer for increasing the sensitivity. When Nd-YAGlasers are used, engraving is as a rule permitted only through the useof IR absorbers. In the case of CO₂ lasers, the engraving speed can beincreased. Suitable absorbers are disclosed in EP-A 640 043 and EP-A 640044 and comprise strongly colored pigments, such as carbon black, orIR-absorbing dyes which are also usually strongly colored.

[0010] The use of strongly colored IR absorbers results in the relieflayers being substantially opaque in the UV/VIS range too. Such layerstherefore cannot be photochemically reinforced or crosslinked since thedepth of penetration of the actinic radiation is extremely limited owingto the very strong absorption. As a solution, EP-B 640 043 thereforeproposes producing a thick layer by casting a multiplicity of thinlayers, followed in each case by photochemical crosslinking of eachindividual layer. However, this procedure is inconvenient and expensive.Moreover, the adhesion between the layers when a further layer is castonto a crosslinked layer is frequently unsatisfactory.

[0011] Laser-engravable flexographic printing elements which have anopaque relief layer can also be produced by casting the layer and thencrosslinking it thermally, for example with the use of monomers andthermal polymerization initiators. However, casting too permits only theproduction of layers having limited thickness since, with increasinglayer thickness, layer defects are also increasingly caused duringevaporation of the solvent. Flexographic printing plates have layerthicknesses of up to 7 mm. Such layer thicknesses are achievable as arule only by means of repeated casting one on top of the other ifhigh-quality layers are to be obtained, and the procedure is accordinglyinconvenient and expensive. Furthermore, many substrate films no longerhave sufficient dimensional stability at the temperatures of thermalcrosslinking.

[0012] It is an object of the present invention to provide a process forthe production of flexographic printing plates, in which the printingrelief is engraved by means of a laser into relief layers which containabsorbers for laser radiation, and in which even thicker layers and anyfurther layers present can be crosslinked in a single operation.

[0013] We have found that this object is achieved by the processdescribed at the outset.

[0014] Regarding the present invention, the following may be statedspecifically.

[0015] For the novel process, an elastomeric relief layer whichcomprises at least one elastomeric binder and at least one absorber forlaser radiation is first applied to a dimensionally stable substrate. Asa rule, the relief layer is opaque.

[0016] Examples of suitable dimensionally stable substrates includefilms of polyethylene terephthalate (PET), polyethylene naphthalate(PEN), polybutylene terephthalate, polyamide or polycarbonate,preferably PET or PEN films. Conical or cylindrical sleeves of saidmaterials can also be used as substrates. Woven glass fiber fabrics orcomposite materials comprising glass fibers and suitable polymericmaterials are also suitable for sleeves. Metallic substrates are ingeneral not suitable for carrying out the process because they heat upexcessively under electron beams, although this should not rule outtheir use in special cases.

[0017] The dimensionally stable substrate can optionally be coated withan adhesion-promoting layer for better adhesion of the relief layer.

[0018] The relief layer comprises at least one elastomeric binder. Thechoice of the binders is limited only in that relief layers suitable forflexographic printing have to be obtained. Suitable binders are chosenby those skilled in the art in accordance with the desired properties ofthe relief layer, for example with regard to hardness, resilience or inktransfer behavior.

[0019] Examples of suitable elastomers include substantially 3 groups,without it being intended to restrict the invention thereto.

[0020] The first group comprises those elastomeric binders which haveethylenically unsaturated groups. The ethylenically unsaturated groupsare crosslinkable by means of electron beams. Such binders are, forexample, those which contain 1,3-diene monomers, such as isoprene orbutadiene, as polymerized units. The ethylenically unsaturated group canon the one hand act as a chain building block of the polymer (1,4incorporation) or can be bonded as a side chain (1,2 incorporation) tothe polymer chain. Examples are natural rubber, polybutadiene,polyisoprene, styrene/butadiene rubber, nitrile/butadiene rubber,acrylate/butadiene rubber, acrylonitrile/isoprene rubber, butyl rubber,styrene/isoprene rubber, polynorbornene rubber orethylene/propylene/diene rubber (EPDM).

[0021] Further examples include thermoplastic elastomeric blockcopolymers of alkenyl aromatics and 1,3-dienes. The block copolymers maybe both linear block copolymers and radial block copolymers. Usually,they are three-block copolymers of the A-B-A type, but they may also betwo-block copolymers of the A-B type or those having a plurality ofalternating elastomeric and thermoplastic blocks, e.g. A-B-A-B-A. Blendsof two or more different block copolymers may also be used. Commercialthree-block copolymers frequently contain certain proportions oftwo-block copolymers. The diene units may be 1,2- and/or 1,4-linked.Both block copolymers of the styrene/butadiene type and of thestyrene/isoprene type may be used. They are commercially available, forexample, under the name Kraton®. Thermoplastic elastomeric blockcopolymers having terminal blocks of styrene and a randomstyrene/butadiene middle block may also be used and are available underthe name Styroflex®.

[0022] Further examples of binders having ethylenically unsaturatedgroups include modified binders in which crosslinkable groups areintroduced into the polymeric molecule by grafting reactions.

[0023] The second group includes those elastomeric binders which havefunctional groups which are crosslinkable by means of electron beams.These are preferably functional side groups. However, they may also begroups which are integrated into the polymer chain. Examples of suitablefunctional groups include —OH, —NH₂, —NHR, —NCO, —CN, —COOH, —COOR,—CONH₂, —CONHR, —CO—, —CHO or —SO₃H, where R is in general an aliphaticor aromatic radical. Protic functional groups, for example —OH, —NH₂,—NHR, —COOH or —SO₃H, have proven particularly advantageous for theproduction of flexographic printing plates by means of electron beamcrosslinking and laser engraving. Examples of binders include acrylaterubbers, ethylene/acrylate rubbers, ethylene/acrylic acid rubbers orethylene/vinyl acetate rubbers and their partly hydrolyzed derivatives,thermoplastic elastomeric polyurethanes, sulfonated polyethylenes orthermoplastic elastomeric polyesters.

[0024] It is of course also possible to use elastomeric binders whichhave both ethylenically unsaturated groups and functional groups.Examples include copolymers of butadiene with (meth)acrylates,(meth)acrylic acid or acrylonitrile, and furthermore copolymers or blockcopolymers of butadiene or isoprene with styrene derivatives havingfunctional groups, for example block copolymers of butadiene and4-hydroxystyrene. Unsaturated thermoplastic elastomeric polyesters andunsaturated thermoplastic elastomeric polyurethanes are likewisesuitable.

[0025] The third group of elastomeric binders includes those which haveneither ethylenically unsaturated groups nor functional groups. Examplesof these are ethylene/propylene elastomers, ethylene/1-alkyleneelastomers or products obtained by hydrogenating diene units, forexample SEBS rubbers.

[0026] It is of course also possible to use mixtures of two or moreelastomeric binders, these being either binders comprising in each caseonly one of the groups described or mixtures of binders comprising twoor all three groups. The possible combinations are limited only insofaras the suitability of the relief layer for flexographic printing may notbe adversely affected by the binder combination. For example, a mixtureof at least one elastomeric binder which has no functional groups withat least one other binder which has functional groups can advantageouslybe used.

[0027] The amount of elastomeric binder or binders in the relief layeris usually from 40 to 99, preferably from 50 to 95, very particularlypreferably from 60 to 90, % by weight, based on the sum of allcomponents.

[0028] The relief layer furthermore comprises at least one absorber forlaser radiation. Mixtures of different absorbers for laser radiation mayalso be used. Suitable absorbers for laser radiation have highabsorption in the range of the laser wavelength. In particular,absorbers which have a high absorption in the near infrared and in thelonger-wave VIS range of the electromagnetic spectrum are suitable. Suchabsorbers are particularly suitable for the absorption of the radiationof powerful Nd-YAG lasers (1 064 nm) and of IR diode lasers, whichtypically have wavelengths of from 700 to 900 nm and from 1 200 to 1 600nm.

[0029] Examples of suitable absorbers for the laser radiation in theinfrared spectral range are strongly absorbing dyes, for examplephthalocyanines, naphthalocyanines, cyanines, quinones, metal complexdyes, such as dithiolenes or photochromic dyes.

[0030] Other suitable absorbers are inorganic pigments, in particularintensely colored inorganic pigments, for example chromium oxides, ironoxides, hydrated iron oxides or carbon black.

[0031] Finely divided carbon black grades having a particle size of from10 to 50 nm are particularly suitable as absorbers for laser radiation.

[0032] Most of the stated laser absorbers also have a high absorption inthe UV and in the VIS range of the electromagnetic spectrum andaccordingly have an intense color. The relief layers which contain theseabsorbers are therefore generally opaque or at least substantiallytranslucent and hence not completely photochemically crosslinkable. Atleast 0.1% by weight, based on the sum of all components of thelaser-engravable relief layer, of absorber is used. The amount of addedabsorber is chosen by a person skilled in the art according to theproperties of the relief layer which are desired in each case. In thiscontext, a person skilled in the art will furthermore take into accountthe fact that the added absorbers influence not only the speed andefficiency of the engraving of the elastomeric layer by laser but alsoother properties of the flexographic printing element, for example itshardness, resilience, thermal conductivity or ink acceptance. As a rule,more than 40% by weight, based on the sum of all components of thelaser-engravable elastomeric layer, of absorbers for laser radiation aretherefore unsuitable. The amount of the absorber for laser radiation ispreferably from 1 to 30, particularly preferably from 5 to 20, % byweight.

[0033] The elastomeric relief layer can optionally also comprise lowmolecular weight or oligomeric compounds crosslinkable by means ofelectron beams. Oligomeric compounds generally have a molecular weightof not more than 20 000 g/mol. Low molecular weight and oligomericcompounds are to be referred to below as monomers for the sake ofsimplicity.

[0034] Monomers may be added on the one hand in order to increase thecrosslinking rate if this is desired by a person skilled in the art.With the use of elastomeric binders from groups 1 and 2, the addition ofmonomers for acceleration is generally not absolutely necessary. In thecase of elastomeric binders from group 3, the addition of monomers is asa rule advisable without being absolutely necessary in every case.

[0035] Regardless of the question of the crosslinking rate, monomers canalso be used for controlling the crosslinking density in the course ofthe electron beam curing and for establishing the desired hardness ofthe crosslinked material. Depending on the type and amount of added lowmolecular weight compounds, more or less dense networks are obtained.

[0036] Monomers used may be, on the one hand, the known ethylenicallyunsaturated monomers which can also be used for the production ofconventional photopolymer flexographic printing plates. The monomersshould be compatible with the binders and have at least oneethylenically unsaturated group. They should not be readily volatile.The boiling point of suitable monomers is preferably not less than 150°C. Amides and esters of acrylic acid or methacrylic acid with mono- orpolyfunctional alcohols, amines, amino alcohols or hydroxyethers andhydroxyesters, styrene or substituted styrenes, esters or fumaric ormaleic acid or allyl compounds are proven [sic] particularly suitable.Examples include butyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate,1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, 1,6-hexanedioldimethacrylate, 1,9-nonanediol diacrylate, trimethylolpropanetriacrylate, dioctyl fumarate and N-dodecylmaleimide.

[0037] It is also possible to use monomers which have at least onefunctional group crosslinkable under the action of electron beam curing.The functional group is preferably a protic group. Examples include —OH,—NH₂, —NHR, —COOH or —SO₃H. Di- or polyfunctional monomers in whichterminal functional groups are linked to one another via a spacer canparticularly preferably be used. Examples of such monomers includedialcohols, for example 1,4 butanediol [sic], 1,6-hexanediol, 1,8octanediol [sic] or 1,9 nonanediol [sic], diamines, for example1,6-hexanediamine or 1,8-hexanediamine, and dicarboxylic acids, forexample oxalic acid, malonic acid, adipic acid, 1,6-hexanedicarboxylicacid, 1,8-octanedicarboxylic acid, 1,10-decanedicarboxylic acid,phthalic acid, terephthalic acid, maleic acid or fumaric acid.

[0038] It is also possible to use monomers which have both ethylenicallyunsaturated groups and functional groups. Examples are ω-hydroxyalkylacrylates, such as ethylene glycol mono(meth)acrylate, 1,4-butanediolmono(meth)acrylate or 1,6-hexanediol mono(meth)acrylate.

[0039] It is of course also possible to use mixtures of differentmonomers, provided that the properties of the relief layer are notadversely affected by the mixture.

[0040] As a rule, the amount of added monomers is from 0 to 30,preferably from 0 to 20, % by weight, based on the amount of allcomponents of the relief layer.

[0041] The elastomeric relief layer may furthermore comprise additivesand assistants, for example dyes, dispersants, antistatic agents,plasticizers or abrasive particles. However, the amount of suchadditives should as a rule not exceed 20% by weight, based on the amountof all components of the elastomeric relief layer of the recordingelement.

[0042] The elastomeric relief layer may also be composed of a pluralityof relief layers. These elastomeric part-layers may be of identical,roughly identical or different composition.

[0043] The thickness of the elastomeric relief layer or of all relieflayers together is as a rule from 0.1 to 7 mm, preferably from 0.4 to 7mm. The thickness is chosen suitably by a person skilled in the art inaccordance with the desired use of the flexographic printing plate.

[0044] The flexographic printing element used as starting material canoptionally furthermore have an upper layer having a thickness of notmore than 100 μm. The composition of such an upper layer can be chosenwith respect to optimum printing properties, for example ink transfer,while the composition of the relief layer underneath is chosen withrespect to optimum hardness or resilience. The thickness is preferablyfrom 5 to 80 μm, particularly preferably from 10 to 60 μm. The upperlayer must either itself be laser-engravable or at least be removable inthe course of the laser engraving, together with the relief layerunderneath. It comprises at least one polymeric binder which need notnecessarily be elastomeric. It can furthermore comprise an absorber forlaser radiation or monomers or assistants.

[0045] The starting material for the process can be prepared, forexample, by dissolving or dispersing all components in a suitablesolvent and pouring onto a substrate. In the case of multilayerelements, a plurality of layers can be cast one on top of the other in amanner known in principle. Since the wet-on-wet method is used, thelayers bind well to one another. An upper layer, too, can be cast ontop. Alternatively, the individual layers can be cast, for example, ontemporary substrates and the layers then united with one another bylamination. After the casting, a cover sheet can optionally also beapplied for protecting the starting material from damage.

[0046] Very particularly advantageously, however, thermoplasticelastomeric binders are used for the novel process, and the productionis carried out in a known manner by extrusion between a substrate filmand a cover sheet or a cover element, followed by calendering, asdisclosed, for example, by EP-A-084 851. In this way, it is possiblealso to produce thick layers in a single operation. Multilayer elementscan be produced by means of coextrusion.

[0047] In process step (b), the relief layer is uniformly crosslinked bymeans of electron beams. If the flexographic printing element still hasa protective film, this should generally be peeled off before thecrosslinking. However, this is not always essential, particularly in thecase of crosslinking by means of electron beams.

[0048] Suitable apparatuses for crosslinking by means of electron beamsare known in principle to a person skilled in the art. The exposure toelectrons can be carried out in line directly after the continuousproduction of the relief layer, for example directly after thecalendering. However, the exposure to electrons can advantageously alsobe carried out in a separate process step.

[0049] During the uniform crosslinking, the flexographic printingelement used as starting material is very uniformly exposed to electronbeams. Ideally, the total surface of the flexographic printing elementshould be absolutely uniformly exposed, although in practice there areof course always certain variations. However, relatively largevariations should be avoided. In order to achieve uniform exposure, theflexographic printing element should be placed as flat as possible onthe supporting surface.

[0050] In the novel process, the flexographic printing elements are as arule exposed only from the top of the elements. However, the presentinvention does of course also include the procedure whereby the elementis exposed from the top and from the bottom.

[0051] The minimum total dose for crosslinking is 40 kGy (1 Gy=1 J/kg).The maximum irradiation dose is established by a person skilled in theart in accordance with the desired properties, for example hardness orrestoring force of the flexographic printing plate. As a rule, however,it is not advisable to use more than 200 kGy for crosslinking and it isparticularly preferable to use not more than 150 kGy for crosslinking. Atotal dose of from 60 to 120 kGy for irradiation has proven useful.

[0052] The energy of the electron beams is determined by a personskilled in the art according to the thickness and composition of theflexographic printing element. Said energy is decisive for the maximumdepth of penetration of the electron beams in the relief layer. In thecase of the relief layers which are used according to the invention andcontain an absorber for laser radiation, it has however generally provenuseful to use electron beams having an energy of at least 2 MeV.

[0053] The exposure to electrons can be carried out in such a way thatthe total dose is administered in a single irradiation process. Thepower of the dose should be very high in order to achieve very shortexposure times. On the other hand, it must not be so high that theflexographic printing element heats up excessively, since otherwise thedimensional stability of the flexographic printing element might beimpaired. Heating up to above 80° C. should be avoided. In order toachieve an optimum result, it is usually advantageous to useparticularly thermally stable substrate films, for example thosecomprising PEN.

[0054] The irradiation is as a rule carried out in air, but in specialcases can of course also be effected under inert gases, such as argon ornitrogen. If desired, the plates to be exposed can also be encapsulatedfor the exclusion of air.

[0055] It is furthermore advantageous to cool the flexographic printingelement during the irradiation, for example by an air stream which ispassed over, or by placing said element on a cooled supporting surface.

[0056] In a particularly advantageous embodiment of the novel process,the total dose of electron beam is distributed over two or morepart-doses. The part-doses may be of equal or different magnitudes andthe electron beams may have the same energy or different energy or thesame or a different power of the dose.

[0057] The individual part-doses can follow directly in succession.However, they may also advantageously be interrupted for irradiationpauses of equal or different length. The irradiation may be interruptedonly briefly or for a longer time. Irradiation pauses of more than 60minutes between the individual doses should however be avoided.Irradiation pauses of from 1 to 30 minutes have proven useful.

[0058] Some embodiments for the crosslinking step by means of electronbeams, which have proven particularly useful, are described in oredetail below.

[0059] In one embodiment for the electron beam crosslinking step, theenergy of the electron beams is identical or virtually identical for alladministered part-doses. After each part-dose, an irradiation pause ismaintained. Irradiation is preferably effected with a relatively highpower of the dose, with the result that the relief layer heats upconsiderably. Temperatures of more than 100° C. should however beavoided. In the irradiation causes, the relief layer may react and coolagain.

[0060] In a further embodiment, the energy of the electron beams in thecase of at least one of the administered part-doses differs from that ofthe other part-doses. For example, the energy of the electron beams ofthe part-doses administered first can be chosen so that the flexographicprinting element is crosslinked through the total depth of the relief,whereas the energy of the electron beams of the part-dose administeredlast is such that further crosslinking is effected only in a thin layerat the surface. It is thus possible to obtain a flexographic printingplate which has a relatively soft lower layer and a comparatively harderupper layer.

[0061] The energy of the electron beams may also differ for allpart-doses. This also permits crosslinking profiles of different types.For example, it is possible to start with the part-dose for which theelectron beams have the highest energy and then to reduce the electronenergy for each further part-dose. In this manner, it is possible toobtain a flexographic printing plate in which the crosslinking densityof the relief layer increases stepwise from the substrate film to theprinting surface.

[0062] It has proven useful in all embodiments to use electron beamshaving an energy of at least 2 MeV, at least in one of the steps.

[0063] In a further embodiment, a plurality of flexographic printingelements can also be stacked one on top of the other to increase theefficiency. In order to achieve uniform crosslinking, it is advisablehere too to effect irradiation in a plurality of part-doses and tochange the sequence of flexographic printing elements cyclically in thestack for each irradiation. It is also possible initially to irradiate acomplete stack once or several times and, in a final step, to harden thesurface for the elements individually in a controlled manner usingelectron beams having a small depth of penetration.

[0064] In process step (c), a printing relief is engraved by means of alaser into the layer crosslinked by means of electron beams.Advantageously, image elements in which the sidewalls of the imageelements initially fall away perpendicularly and do not broaden untilthe lower region of the image elements are engraved. A good shouldershape in combination with a small increase in tonal value is thusachieved. However, it is also possible to engrave dot sidewalls ofanother shape.

[0065] IR lasers are particularly suitable for laser engraving. However,it is also possible to use lasers having shorter wavelengths, providedthat the laser has sufficient intensity. For example, afrequency-doubled (532 nm) or frequency-tripled (355 nm) Nd-YAG laser oreximer [sic] laser (e.g. 248 nm) can also be used. If required forremoval of material, absorbers for laser irradiation which areappropriately adapted to the laser wavelength to be used in each casemust be used.

[0066] For example, a CO₂ laser having a wavelength of 10 640 nm can beused for laser engraving. Lasers having a wavelength of from 600 to 2000 nm are particularly advantageously used. For example, Nd-YAG lasers(1 064 nm), IR diode lasers or solid-state lasers can be used. Nd-YAGlasers are particularly preferred for carrying out the novel process.The image information to be engraved is transmitted directly from thelayout computer system to the laser apparatus. The lasers can beoperated either continuously or in a pulsed manner.

[0067] As a rule, the flexographic printing plate obtained can be useddirectly. If desired, however, the flexographic printing plate obtainedcan also be subsequently cleaned. As a result of said cleaning step,layer components which have become detached but may not have beencompletely removed from the plate surface are removed. As a rule, simpletreatment with water, water/surfactant or alcohol is entirelysufficient.

[0068] The novel process can be carried out in a single productionoperation in which all process steps are carried out in succession.Advantageously, however, the process can also be interrupted afterprocess step (b). The crosslinked, laser-engravable recording elementcan be made up and stored and further processed only later by means oflaser engraving to give a flexographic printing plate or flexographicsleeve. It is advantageous here to protect the flexographic printingelement, for example with a temporary cover sheet, for example of PET,which of course has to be peeled off again before the laser engraving.

[0069] The novel process has a number of important advantages over theprior art:

[0070] It permits the production of flexographic printing plates whoserelief layers comprise absorbers for laser radiation also with largelayer thickness and high quality. Only one operation is required for thecrosslinking.

[0071] In the course of the electron beam crosslinking, the adhesionbetween the substrate film and the relief layer is also substantiallyimproved. The same applies to the adhesion between an optionally presentupper layer and the relief layer.

[0072] The division of the total radiation dose into a plurality ofpart-doses whose electron beams have different energies makescrosslinking profiles accessible in a simple manner. In this way, forexample, flexographic printing elements having a hardened surface can beobtained. Hardened surfaces have the advantage that no fusion edges areformed around the engraved relief elements during engraving by means oflasers. Fusion edges give rise to impairment of the printed image duringprinting. Furthermore, such plates have high abrasion resistance.

[0073] The thermal stress on the flexographic printing element in thecourse of the crosslinking can be substantially reduced in comparisonwith thermal crosslinking or even virtually completely avoided. Thisleads to flexographic printing plates having substantially improveddimension stability and hence to substantially better printing quality.

[0074] The examples which follow illustrate the invention.

EXAMPLE 1

[0075] A relief layer comprising a binder having ethylenicallyunsaturated groups was produced. The following components were used forthe relief layer. Amount [% Components Starting materials by wt.] BinderPolybutadiene rubber (high 68.5 vinyl content) Absorber for Finelydivided carbon black 10.0 laser radiation Monomers Lauryl acrylate 10.0Additives Polybutadiene oil (plasticizer) 10.0 Heat stabilizer 1.5

[0076] Binder, additives and absorber for laser radiation were mixed ina laboratory kneader at a material temperature of 150° C. After 15minutes, the absorber for laser radiation had been homogeneouslydispersed. The compound thus obtained was dissolved together with themonomer at 80° C. in toluene, cooled to 60° C. and cast onto anuncoated, 125 μm thick PET film. After drying in air for 24 hours atroom temperature and drying for 3 hours at 60° C., the relief layerobtained (layer thickness 900 μm) was laminated with a second, 125 μmthick PET film coated with a mixture of adhesive-forming components.Before the further treatment, the element was stored for 1 week at roomtemperature.

EXAMPLE 2

[0077] A relief layer comprising a binder mixture having ethylenicallyunsaturated groups was produced. The following components were used forthe relief layer. Amount Components Starting materials [% by wt.]Binders EPDM rubber comprising 75.5 5% by weight of ethylidenenorborneneas a termonomer Polybutadiene rubber (high 4.0 vinyl content Absorberfor Finely divided carbon black 10.0 laser radiation Monomers Laurylacrylate 7.5 Trimethylolpropane 1.5 trimethacrylate Additives Heatstabilizer, dispersant 1.5

[0078] Binders, additives and absorber for laser radiation were mixed ina laboratory kneader at a material temperature of 170° C. After 15minutes, the absorber for laser radiation had been homogeneouslydispersed. The compound thus obtained was dissolved together with themonomers at 80° C. in toluene, cooled to 60° C. and cast onto anuncoated, 125 μm thick PET film. After drying in air for 24 hours atroom temperature and drying for 3 hours at 60° C., the relief layerobtained (layer thickness 800 μm) was laminated with a second, 175 μmthick PET film coated with a mixture of adhesive-forming components.Before the further treatment, the element was stored for 1 week at roomtemperature.

EXAMPLE 3

[0079] A relief layer comprising a binder having ethylenicallyunsaturated groups was produced by means of extrusion and subsequentcalendering between a cover sheet and substrate film. The followingcomponents were used for the relief layer. Amount [% Components Startingmaterials by wt.] Binder SIS three-block copolymer 80.0 comprising 15%by weight of styrene (Kraton D-1161, from Kraton Polymers) Absorber forlaser Finely divided carbon black 6.0 radiation Monomers Hexanedioldiacrylate 6.0 Hexanediol dimethacrylate 6.0 Additives Heat stabilizer,antiozonant wax 2.0

[0080] The components were thoroughly mixed with one another in atwin-screw extruder at a material temperature of 140-160° C., extrudedthrough a slot die and then calendered between a cover sheet andsubstrate film. The thickness of the relief layer was 860 μm. Before thefurther treatment, the element was stored for 1 week at roomtemperature.

EXAMPLE 4 (COMPARATIVE EXAMPLE)

[0081] A relief layer comprising a binder having ethylenicallyunsaturated groups was produced by means of extrusion and subsequentcalendering between a cover sheet and a substrate film. The followingcomponents were used for the relief layer. Amount [% Components Startingmaterials by wt.] Binder SIS three-block copolymer 79.0 comprising 15%by weight of styrene (Kraton D-1161, from Kraton Polymers) Absorber forFinely divided carbon black 6.0 laser radiation Photoinitiator Benzildimethyl ketal 1.0 Monomers Hexanediol diacrylate 6.0 Hexanedioldimethacrylate 6.0 Additives Heat stabilizer, antiozonant wax 2.0

[0082] The components were thoroughly mixed with one another in atwin-screw extruder at a material temperature of 140-160° C., extrudedthrough a slot die and then calendered between a cover sheet and asubstrate film. The thickness of the relief layer was 850 μm. Before thefurther treatment, the element was stored for 1 week at roomtemperature.

[0083] Electron Beam Crosslinking

[0084] An electron beam apparatus (nominal power about 150 kW) which canproduce electron beams having electron energies of 2.5-4.5 MeV was usedfor the crosslinking. The elements to be exposed to the electron beamswere transported through the electron irradiation zone by means ofaluminum pallets which were freely suspended vertically and wereconnected to a guided conveyor belt by means of a mobile suspension sothat uniform transport of the aluminum pallets through the electronirradiation zone could be effected by controlling the conveyor beltspeed.

[0085] Crosslinking by Exposure to UV-A Light

[0086] For crosslinking by exposure to UV-A light, the elements to becrosslinked were exposed for a specific, predetermined time in an F IIIexposure unit from BASF Drucksysteme GmbH under reduced pressure.

[0087] For this purpose, the protective cover sheet of the relevantelement was first removed and a transparent, UV-permeable non-tackyrelief film was then placed on the element to be exposed, in order toprevent adhesion of the element surface to the vacuum film. After theelement to be exposed had been covered with the vacuum film and thereduced pressure had been switched on, the element was exposed uniformlyto UV light for the specified duration.

EXAMPLE 5

[0088] A total of 6 elements according to example 1 were used, of which1 element was retained as a reference (sample No. 0). The energy of theelectron beams was about 3.0 MeV. A gradual irradiation seriescomprising 5 identical part-doses of 20 kGy each was carried out. Thewaiting time between 2 part-doses was 20 minutes in each case. Aftereach part-dose, an element was removed from the irradiation loop and theremaining elements were turned through 180° C. before administration ofthe next part-dose.

[0089] The table below shows the properties of the resultingflexographic printing element as a function of the irradiation dose.Mech. Swelling Gel hardness Part-dose Total dose in toluene* content^(#)(DIN 53505) No. [kGy] [kGy] [% by wt.] [% by wt.] [Shore A] 0 — — ∞  0 120 20 447 77 72 2 20 40 266 86 74 3 20 60 205 91 78 4 20 80 180 93 80 520 100  180 94 81

EXAMPLE 6

[0090] A total of 9 elements according to example 2 were used, of which1 element was retained as a reference (sample No. 0). The energy of theelectron beams was about 3.0 MeV. A gradual irradiation seriescomprising 8 part-doses, some of which differed, was carried out. Thespecific part-doses were in succession 23, 22, 22, 35, 42, 30, 30 and 29kGy. The waiting time between 2 part-doses was 20 minutes in each case.After each part-dose, an element was removed from the irradiation loopand the remaining elements were turned through 180° beforeadministration of the next part-dose.

[0091] The table below shows the properties of the resultingflexographic printing element as a function of the irradiation dose.Mech. Total Swelling hardness Part-dose dose in toluene* Gel content^(#)(DIN 53505) No. [kGy] [kGy] [% by wt.] [% by wt.] [Shore A] 0 — — ∞  0 123  23 444 90 72 2 22  45 274 94 72 3 22  67 199 96 72 4 35 102 167 9873 5 42 144 157 97 74 6 30 174 162 97 74 7 30 204 129 98 74 8 29 233 12198 74

Example 7

[0092] A total of 9 elements according to example 3 were used, of which1 element was retained as a reference (sample No. 0). The energy of theelectron beams was about 3.0 MeV. A gradual irradiation seriescomprising 8 part-doses, some of which differed, was carried out. Thespecific part-doses were in succession 23, 22, 22, 35, 42, 30, 30 and 29kGy. The waiting time between 2 part-doses was 20 minutes in each case.After each part-dose, an element was removed from the irradiation loopand the remaining elements were turned through 180° beforeadministration of the next part-dose.

[0093] The table below shows the properties of the resultingflexographic printing element as a function of the irradiation dose.Mech. Swelling Gel hardness Part-dose Total dose in toluene* content^(#)(DIN 53505) No. [kGy] [kGy] [% by wt.] [% by wt.] [Shore A] 0 — — ∞  0 123  23 ∞  0 39 2 22  45 828 77 52 3 22  67 430 87 58 4 35 102 431 89 635 42 144 331 92 65 6 30 174 322 93 67 7 30 204 260 94 68 8 29 233 260 9468

EXAMPLE 8 (COMPARATIVE EXAMPLE)

[0094] A total of 6 elements according to example 4 were used, of which1 element was retained as a reference (sample No. 0). An irradiationseries with UVA light was carried out as described above using thefollowing individual exposure times: 1, 5, 15, 30 and 60 min.

[0095] The table below shows the properties of the resultingflexographic printing element as a function of the UVA exposure time.Duration Mech. of the UVA Swelling in Gel hardness exposure toluene*content^(#) (DIN 53505) No. [min] [% by wt.] [% by wt.] [Shore A] 0 0 ∞0 1 1 ∞ 0 32 2 5 ∞ 0 33 3 15 ∞ 1 35 4 30 ∞ 3 36 5 60 ∞ 2 34

[0096] Laser Engraving of the Irradiated Flexographic Printing Elements:

[0097] The irradiated flexographic printing elements obtained wereengraved using a CO₂ laser (from ALE, Meridian Finesse, 250 W, engravingspeed =200 cm/s) and an Nd-YAG laser (from ALE, Meridian Finesse, 100 W,engraving speed =100 cm/s). A test pattern consisting of solid areas andvarious line work was engraved into the respective flexographic printingelement. The line work measuring 1 cm×1 cm in each case consisted ofparallel, individual negative lines having an identical line width andidentical line spacing per line element. A list of the engraved linework is shown in the table below. Line Width of the Spacing of theelement negative lines negative lines No. [μm] [μm] 1  20  20 2  40  403  60  60 4  80  80 5  100  100 6  200  200 7  500  500 8 1000 1000

[0098] The quality of the laser-engraved flexographic printing elementwas assessed with the aid of an optical microscope which has a means formeasuring distances or heights and depths.

[0099] For this purpose, the gravure depth was measured in the uniformlyengraved part. Furthermore, the finest line work for which the engravedindividual lines were completely resolved from one another under themicroscope was determined. The individual lines were assessed as beingcompletely resolved from one another if the surface of the positive lineelement remaining between the negative line had a width of at least 5 μmand this surface had the same height as the unengraved parts of thepositive solid area within a difference of 20 μm. In this method ofassessment, a low number for the finest line element still reproducedaccordingly corresponds to good gravure quality, whereas a high numbercorresponds to a lower resolution and hence poorer gravure quality.

[0100] Finally, in particular fusion edges and deposits in the edgezones of the negative elements and solid areas were visually assessed.Engrav- Finest Type of Cross Fusion ing line Ex. cross linking Laseredges depth element No. linking conditions type (visually) [μm] [No.] 5EB  60 kGy CO₂ Few 760 3 5 EB  80 kGy CO₂ None 830 1 5 EB  60 kGy Nd-YAGFew 810 2 5 EB  80 kGy Nd-YAG None 830 1 6 EB  67 kGy CO₂ Moderate 640 36 EB 102 kGy CO₂ Few 700 2 6 EB  67 kGy Nd-YAG Moderate 660 3 6 EB 102kGy Nd-YAG Few 690 2 7 EB 102 kGy CO₂ Moderate 650 2 7 EB 144 kGy CO₂None 710 2 7 EB 102 kGy Nd-YAG Moderate 660 2 7 EB 144 kGy Nd-YAG None680 1 8 UVA  15 min CO₂ Very 390 7 pronounced 8 UVA  60 min CO₂Pronounced 480 5 8 UVA  15 min Nd-YAG Very 430 6 pronounced 8 UVA  60min Nd-YAG Very 450 5 pronounced

[0101] Examples No. 5 to 7 show that fine relief elements of goodquality and without pronounced fusion phenomena can be reproduced usingthe novel laser-engravable flexographic printing elements, in contrastto comparative example No. 8. Moreover, a greater gravure depth issurprisingly achieved using the novel flexographic printing elementsthan with a laser-engravable flexographic printing element according tothe prior art (comparative example No. 8).

[0102] In addition, all electron beam crosslinked flexographic printingelements according to example No. 7 surprisingly have substantiallygreater adhesion to the substrate than the UV-crosslinked flexographicprinting elements according to comparative example No. 8.

We claim:
 1. A process for the production of flexographic printingplates by means of laser engraving, comprising the following steps: a)application of at least one elastomeric relief layer to a dimensionallystable substrate, the relief layer comprising at least one elastomericbinder and at least one absorber for laser radiation, b) uniformcrosslinking of the relief layer, c) engraving of a printing relief intothe crosslinked relief layer by means of a laser, wherein the uniformcrosslinking is carried out by means of electron beams in a minimumtotal dose of 40 kGy.
 2. A process as claimed in claim 1, wherein, in astep (a′), an upper layer having a thickness of not more than 100 μm isfurthermore applied, the upper layer comprising at least one polymericbinder.
 3. A process as claimed in claim 1 or 2, wherein the electronbeams have an energy of at least 2 MeV.
 4. A process as claimed in claim1 or 2, wherein the total dose of electron beams is distributed over twoor more part-doses.
 5. A process as claimed in claim 4, wherein theirradiation is stopped for an irradiation pause after the administrationof any part-dose.
 6. A process as claimed in claim 4 or 5, wherein theenergy of the electron beam is identical for each of the administeredpart-doses.
 7. A process as claimed in claim 4 or 5, wherein the energyof the electron beam for at least one of the administered part-dosesdiffers from that of the other part-doses.
 8. A process as claimed inclaim 4 or 5, wherein the energy of the electron beam differs for alladministered part-doses.
 9. A process as claimed in claim 8, wherein theinitial part-dose is the one in which the electron beam has the highestenergy, and the energy for each further part-dose decreases stepwise.10. A process as claimed in any of claims 4 to 8, wherein at least oneof the part-doses has an energy of at least 2 MeV.
 11. A process asclaimed in any of claims 1 to 10, wherein a total dose of 200 kGy is notexceeded.
 12. A process as claimed in any of claims 1 to 10, wherein atotal dose of 150 kGy is not exceeded.
 13. A process as claimed in anyof claims 1 to 12, wherein the irradiation is carried out usingelectrons in air.
 14. A process as claimed in any of claims 1 to 13,wherein the elastomeric binder has ethylenically unsaturated groups. 15.A process as claimed in any of claims 1 to 13, wherein the elastomericbinder has functional groups crosslinkable under the action of electronbeams.
 16. A process as claimed in claim 15, wherein the functionalgroups are protic groups.
 17. A process as claimed in any of claims 1 to13, wherein the elastomeric binder has ethylenically unsaturated groupsand functional groups crosslinkable under the action of electron beams.18. A process as claimed in any of claims 1 to 13, wherein a mixture ofat least one elastomeric binder which has no functional groups with atleast one further binder which has functional groups is used.
 19. Aprocess as claimed in any of claims 1 to 18, wherein the relief layerfurthermore comprises at least one low molecular weight or oligomericcompound crosslinkable by means of electron beams.
 20. A process asclaimed in claim 19, wherein the low molecular weight compound is anethylenically unsaturated monomer.
 21. A process as claimed in any of[sic] claim 19, wherein the low molecular weight or oligomeric compoundis a compound having functional groups.
 22. A process as claimed in anyof [sic] claim 21, wherein the functional groups are protic groups. 23.A process as claimed in any of claims 1 to 22, wherein the elastomericbinder is a thermoplastic elastomeric binder and the relief layer isproduced by extrusion followed by calendering.
 24. A process as claimedin any of claims 1 to 23, wherein the relief layer is opaque.
 25. Aprocess as claimed in any of claims 1 to 24, wherein the laser engraving(c) is carried out using a laser having a wavelength of 600-2 000 nm.26. A process as claimed in claim 25, wherein the laser engraving (c) iscarried out using an Nd-YAG laser.
 27. A flexographic printing plateobtainable as claimed in any of claims 1 to 26.